Shaari et al. Progress in Earth and Planetary Science (2020) 7:46 Progress in Earth and https://doi.org/10.1186/s40645-020-00362-y Planetary Science

RESEARCH ARTICLE Open Access Sedimentation and sediment geochemistry in a tropical mangrove channel meander, Sungai Kerteh, Peninsular Malaysia Hasrizal Shaari1,2* , Qatrunnada Mohd Nasir1, Hui-Juan Pan3,4*, Che Abd Rahim Mohamed5, Abdul Hafidz Yusoff6, Wan Mohd Afiq Wan Mohd Khalik1, Erick Naim2, Riza Yuliratno Setiawan7 and Edward J. Anthony8,9

Abstract Tropical mangrove swamps are commonly characterized by dense networks of tidal channels that may show pronounced meandering and dendritic patterns. Channel meanders are sometimes accompanied by cut-offs, and, like classical fluvial meanders, record changes in hydrology and sedimentation over time. Channel meandering can, thus, be an important process that contributes to spatial and temporal variability in the preserved record of the sedimentology and geochemistry of mangrove sediments. The aim of this study is to highlight changes in channel meander sedimentation in response to a meander cut-off in a tropical mangrove swamp. Two short sediment cores were sampled, respectively from a point bar (core KR1, 122 cm) at the junction with the neck cut-off and inside the cut-off (core KR2, 98 cm) in the Sungai Kerteh mangroves of Peninsular Malaysia. The profile comparison was based on sediment characteristics, total organic carbon (TOC), and selected elements (Fe, Na, Mg, Mn, Ba, and Sr). A smaller standard deviation of mean grain size (MGS) was found at the point bar (4.37 ± 0.51 ϕ) than in the cut-off (4.43 ± 1.76 ϕ), indicating a difference in flow velocity between the two settings. In turn, these changes in grain size influence channel meander evolution via associated changes in TOC and heavy metals. In order to clarify these relationships, we used principal components analysis and factor analysis. An increased accumulation of selected elements and TOC at the cut-off site from a depth of ~ 60 cm to the core-top segment was probably associated with a slowing down of sediment settling. A higher TOC recorded in the cut-off (2.74 ± 1.42%) compared to the point bar (1.14 ± 0.46%) suggests a propensity for prolonged in situ accumulation of organic matter in the abandoned meander bend. This study provides grain size and sediment geochemical information that is consistent with patterns of active and inactive sedimentation in the meander bends of mangrove channels. Keywords: Mangrove, Tidal channel meander, Mangrove geochemistry, Mangrove sedimentation

1 Introduction and, as noted by Allen (2000) for saltmarsh systems, they Tidal channels in mangroves are important pathways for reflect several linked natural factors that are not well the transport of sediments, dissolved oxygen, nutrients, known, especially in comparison to the better known seedlings, and organic matter (Woodroffe 1992; Anthony fluvial networks. One source of complexity is the bi- 2009). Tidal network types and densities are highly vari- directional tidal flow, in comparison to the unidirec- able, ranging from meandering to dendritic patterns, tional flow in fluvial systems. As in the case of classical fluvial meanders (Leopold and Wolman 1960; Ferguson * Correspondence: [email protected]; [email protected] 1984; Howard 2009), however, mangrove channel mean- 1 Faculty of Science and Marine Environment, Universiti Malaysia Terengganu, ders are expected to record changes in channel hydrol- 21030 Kuala Nerus, Terengganu, Malaysia 3Institute of Earth Science, College of Ocean Science and Resource, National ogy and sedimentation over time. Meandering is formed Taiwan Ocean University, Keelung, Taiwan by river flow momentum alternations from side to side Full list of author information is available at the end of the article

© The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Shaari et al. Progress in Earth and Planetary Science (2020) 7:46 Page 2 of 11

within a channel, commonly across the floodplain, but the Avicennia sp. A major morphological feature of the area processisalsoinherenttoshifting channels within a valley is the Kerteh River, a coastal river approximately 23 km (Edwards and Smith 2002). Meanders are dynamic systems long that originates in hilly terrain at ~ 350 m above sea that can be far from equilibrium, driven by complex linear level, before running through the mangroves and enter- and nonlinear processes (Argyris et al. 1994). Theoretically, ing the sea just north of Kerteh Town. The Kerteh re- the flow erodes sediment at the outside of a bend as it ceives runoff from a number of smaller upstream reaches a maximum velocity. The eroded materials are de- tributaries. The lower reaches of the Kerteh are influ- posited as a point bar on the inside of the bend. These pro- enced by semi-diurnal tides. The tidal range in the area cesses can lead to obstruction of flow at the neck, resulting is mesotidal, ranging from 0.2 m at low tide to 3.3 m at eventually in the formation of a new shorter channel, a me- high tide during spring tides (PHN 2019). There are no ander cut-off, and neck abandonment. Cut-offs have been data on water discharge of the Kerteh River, which is considered as having an important role in shaping river me- modulated seasonally by Monsoon rainfall (mean annual: ander landscapes by leading to the isolation of meander ~ 2800 mm) and at shorter timescales by the spring- bends and perturbing the local dynamics (Hooke 1977;Cam- neap tidal range. poreale et al. 2008; Schwenk and Foufoula-Georgiou 2016). The tidal reaches of the Kerteh River display promin- In mangroves and saltmarshes, entrenched tidal channel net- ent meanders typical of mature mangrove swamps that works are deemed to evolve very slowly through processes are probably characterized by low substrate sedimenta- that include meander-bend erosion and sedimentation, with tion rates, as suggested by the morphological stability of subsequently, very slow change of these inherited forms over the channels. Locally, however, this stability has been af- multi-decadal to centennial timescales (Anthony 2009). Un- fected or enhanced by embankments associated with vil- derstanding these processes in mangrove forests can contrib- lages and communities located on the channel banks. ute to an understanding of changing present environments, According to the classification by Jackson (1978), the under natural and human-induced processes that drive man- present Kerteh channel can be considered as muddy grove sedimentation, as well as of the paleo-environmental fine-grained, with a low width/depth ratio, steep point record (e.g., França et al. 2015; Cohen et al. 2016; Woodroffe bar slopes, and prominent levees. The study site is a et al. 2016). However, elucidation of these processes and the mangrove swamp comprising a meander with a cut-off eventual messages that can be deduced regarding environ- upstream of Kampung Gelugor (Fig. 1). The abandoned mental changes can only be achieved through archives pro- meander segment displays a straight, nearly 1 km-long vided by sediment cores (e.g., Ellison and Stoddart 1991; cut-off channel. Sediments were sampled at two different Ellison and Farnsworth 1997). Such sedimentary archives locations in the mangroves (Fig. 1): the neck cut-off provide a record of the type, rates, and patterns of sedimen- (core KR1) and the inside of the meander (core KR2). tation. Access to such information requires, however, both a The KR1 site is located at the confluence of two tidal classical sedimentological approach as well as geochemical channels, one of which included the cut-off reach with characterization. Potential differences in sedimentation on sampling site KR2. The dominant species at KR1 is Rhi- meander convex banks and correlative erosion on meander zophora apiculata whereas KR2 consists of mixed Avi- concave banks have been evoked to explain differences in cennia sp. and Rhizophora apiculata. the vigor and health of mangroves that are generally clearly visible on aerial photographs and satellite images (Anthony 2.2 Sampling and sample pre-treatment 2004). We know of no work, however, that has been under- The two cores were collected at low tide by driving a taken to highlight the impact of channel meander evolution 2.5 m polyvinyl chloride (PVC) tube with a diameter of on mangrove sedimentation and on the sediment geochem- 6.5 cm into the mangrove sediment. A tight cable and istry. In this paper, we document changes in mangrove chan- rope were attached to the middle part of the PVC tube nel meander sedimentation associated with a meander cut- in order to retrieve it from the mangrove. A cork and off in a Malaysian mangrove system (Fig. 1) using grain-size plastic were used to cover the top of the tube neatly to and geochemical characteristics of core sediments in the create a vacuum condition, enabling slow and cautious vicinity of an abandoned meander. We show that such sedi- recovery of the cores. The pull-out process was con- mentological and geochemical characterization can be used ducted carefully to reduce loss of sample material and to in identifying change in mangrove environments. maintain the lithological structure of the core sediment. The collected core samples were immediately processed 2 Methods/experimental in order to reduce the compaction effects. The lengths 2.1 Study site of core KR1 and core KR2 were 122 cm and 98 cm, re- The mangroves of Sungai Kerteh are located in Kema- spectively. Samples from both cores were cut and split man District, Terengganu, Peninsular Malaysia (Fig. 1). into two segments. Each segment was sliced at 2 cm in- The dominant mangroves are Rhizophora apiculata and tervals, yielding a total of 61 samples for KR1 and 49 Shaari et al. Progress in Earth and Planetary Science (2020) 7:46 Page 3 of 11

Fig. 1 Map showing the Kerteh mangrove forest in Peninsular Malaysia and the locations of cores KR1 (4° 31′ 27. 5" N, 103° 26′ 25.91" E) and KR2 (4° 31′ 10.98" N, 103° 26′ 19.68" E). The white arrows indicate the river flow directions of main water body of Sungai Kerteh from upstream toward the tidal channel and pink arrows indicate the river flow direction from small branch of Sungai Kerteh. The blue dash box indicates the cutoff location and the pale green dash box indicates new mangrove vegetation at the old river’s pathway due to the active sedimentation process (source: Google Earth) samples for KR2. In order to avoid contamination, the were analyzed using a laser diffraction analyzer (MALV samples were stored in clean-labeled vinyl bags in the la- ERN Mastersizer 2000). All data collected were sub- boratory before analyses of grain size, metal content, and jected to statistical analysis using the moments statistical total organic carbon were carried out. The samples were method (Friedman and Johnson 1982). Grain-size char- oven dried at 50 °C and powdered using an agate mortar acteristics (particle mean grain size and sorting) were with a pestle for further analysis except for sediment calculated using the formula of Folk and Ward (1957). characteristics and grain size. The relative proportions of sand, silt, and clay were de- termined using the textural triangle proposed by USDA 2.3 Sediment characteristics (1987). Approximately 5 g of the sediment samples were trans- ferred into a 100 ml beaker and diluted with distilled 2.4 Total organic carbon water as a medium for the digestion process. A few The total organic carbon was determined based on the drops of hydrochloric acid (HCl) and 15% of hydrogen Walkley-Black chromic acid wet oxidation method peroxide (H2O2) were added into the beaker to remove (Walkley and Black 1934; Allison and Moodie 1965). carbonate content and organic materials. Calgon solu- Approximately, 10 ml of potassium dichromate was tion (20% sodium thiosulphate) was added and acted as added to 0.5 g of sample in a test tube. Then, 20 ml of an agent to disperse the samples into single particles. sulfuric acid was added and left in a beaker of boiling The samples were left at room temperature for 24 h to water (30 min). The test tube was rinsed with 200 ml of allow complete particle dispersion. Sediment samples distilled water into a conical flask and 10 ml of Shaari et al. Progress in Earth and Planetary Science (2020) 7:46 Page 4 of 11

phosphoric acid with a 1 ml diphenylamine indicator Table 1 The value of accuracy analysis for standard reference of added. The solution was mixed until it turned dark blue. trace elements Finally, ferum (II) sulfate was titrated until the solution Trace element Certified value Analysis value Percentage (%) turned green and the total volume of ferum (II) sulfate Fe (%) 2.01 ± 0.04 2.20 ± 0.10 109.5 was recorded. The calculation of organic carbon content Mg (%) 0.40 ± 0.01 0.39 ± 0.27 97.5 was as follows: Mn (mg/kg) 234.5 ± 2.80 204.0 ± 17.7 86.99 TOC (%) = (V1−V2) × 0.03 × 100/sample weight (g) where, Sr (mg/kg) 68 65.2 95.88 V1 = volume of potassium dichromate (ml) V2 = volume of ferum (II) sulfate (ml) software Minitab version 17 (Minitab Inc., State College, A set of blanks and standard research material (SRM) USA). dextrose monohydrate was applied in every batch of sample analysis in order to determine the background 3 Results and discussion content of organic carbon and as a control on the ana- 3.1 Particle mean size and sorting, and sediment facies lysis. The standard percentage value of total organic car- Figure 2a shows the mean grain size (MGS) of sediment bon in dextrose monohydrate is 36%, while the average against depth for cores KR1 and KR2. Values ranged percentage of measured dextrose monohydrate of the from 3.47 ϕ-5.92 ϕ for KR1 and 1.52 ϕ-6.47 ϕ for KR2. analysis was 36 ± 1.2%. The crosscheck with three repli- The highest and lowest MGS values in KR1 were re- cates at every 10 cm core depth for both core samples corded at depths of 32 cm and 52 cm, respectively. For was also set up in the analysis to ensure the results are KR2, the highest and lowest values occurred at depths of credible and acceptable. 56 cm and 82 cm. The profile of MGS in KR2 showed a significant change at 60 cm depth. Figure 2b shows the 2.5 Metal analysis sorting values of sediment in both cores. KR1 ranged be- Sample digestion followed the published methods of Nor- tween 0.44 (moderately sorted) and 2.61 (moderately iki et al. (1980) and Kamaruzzaman (1999)withthemodi- well-sorted). The values of KR2 were between 1.60 and fication of mixed acid ratio and digesting temperature. 2.30 (moderately sorted and moderately well-sorted). Approximately, 0.05 g of homogenized samples were The average value in KR2 was 1.60 ± 2.34 Ø with the digested in a concentrated mixed acid ratio of HF, HNO3, highest value at 66 cm (2.61 Ø) and the lowest at 0.44 Ø. and HCl (2: 3.5: 3.5) in a sealed Teflon vessel at 100 °C for In KR1, the mean was 0.44 ± 2.61 Ø with the highest 7 h. After cooling to room temperature, a clear digested value at a depth of 32 cm (2.34 Ø) and the lowest value solution was transferred into a 15 ml polypropylene test at 102 cm (1.60 Ø). The best-sorted sediment was re- tube and diluted with deionized water. An inductively corded at a depth of 80 cm in core KR1. As in the case coupled plasma mass spectrometer (ICP-MS) was used for of MGS, sorting also showed the same trend with depth. the precise determinations of selected heavy metals (Fe, Figure 3 shows a textural classification plot of sediment Mg, Cu, Mn, Ba, and Sr). The accuracy of the analytical in the two cores. The sediments in KR1 from the point procedure was assessed by analyzing standard research bar exhibit a high percentage of sand (40 to 80%) in con- material in duplicate from the National Bureau of Stand- trast to sediment from KR2 from the meander cut-off ard (NBS) 1646a. The recovery test coincided with the which is less sandy (10 to 40%). certified values of NBS 1646a. The recovery percentage of measured metals was found to be acceptable, ranging be- 3.2 Total organic carbon tween 86.99% and 109.50% (Table 1). The TOC was determined in KR1 and KR2 (Fig. 2c). The TOC content was significantly higher in core KR2 2.6 Principal components analysis and factor analysis ranging from 0.6-5.5% as compared to core KR1 with Principal components analysis and factor analysis were values of 0.24-4.26%. The average TOC values in core used to highlight relationships among the variables. The KR2 and KR1 were 2.74% ± 1.42% and 1.14% ± 0.46 % main component was extracted using principal compo- (Fig. 2c). There were two obvious trends between TOC nent analysis through the eigen-decomposition method. contents of cores KR1 and KR2. Like the metal and Factor analysis was used to explain the latent variables MGS trends, there were two apparent trends for TOC rendered in the data sets using Varimax method after content in both core samples. The TOC content fluctu- Kaiser normalization. Factor loading scores were catego- ated from the bottom of the core toward the surface rized as follows: VF > 0.75 (strong), 0.75 < VF > 0.5 with no significant changes for core KR1. On the other (moderate), and < 0.50 (weak), corresponding to the ab- hand, the content in core KR2 was only comparable to solute varifactor values (Liu et al. 2003). The statistical that of core KR1 at depths between 98 cm and 76 cm. data processing and analysis were generated by using the The concentration of TOC drastically increased at a Shaari et al. Progress in Earth and Planetary Science (2020) 7:46 Page 5 of 11

Fig. 2 Vertical profile of (a) mean grain size, (b) sorting, and (c) TOC

Fig. 3 Textural classification of sediment from KR1 (red dots) and KR2 (blue dots) Shaari et al. Progress in Earth and Planetary Science (2020) 7:46 Page 6 of 11

depth of 72 cm and remained high up to the surface of ± 0.30%, 1.21 ± 0.77%, 057.92 mg/kg, 63.01 ± 15.49 mg/ the core. kg, 17.48 ± 2.14 mg/kg, and 9.1 ± 1.5 mg/kg. The elem- ent concentrations in core KR2 varied from 0.07 to 3.3 Metals 2.24% for Fe, 0.30 to 2.14% for Na, 0.01 to 0.71% for Mg, The element concentrations in core KR1 varied from 5.34 to 173.80 mg/kg for Mn, 3.20 to 34.20 mg/kg for Ba, 0.32 to 2.18% (Fe), 0.51 to 2.38% (Na), 0.49 to 0.95% and 0.50 to 6.52 mg/kg for Sr. The average concentra- (Mg), 81.60 to 204.00 mg/kg (Mn), 22.40 to 47.60 mg/kg tions of elements in this study followed the order of Fe > (Ba), and 1.37 to 7.98 mg/kg (Sr) (Fig. 4). The average Na > Mg > Mn > Ba > Sr. In general, the selected elem- concentrations of Fe, Na, Mg, Mn, Ba, and Sr were 1.25 ent content in KR1 did not show an abrupt change from

Fig. 4 Selected metals concentrations in cores KR1 and KR2 Shaari et al. Progress in Earth and Planetary Science (2020) 7:46 Page 7 of 11

the bottom toward the surface. However, the element between the two cores are less manifest in sorting, with content in the core KR2 showed a significant increase at the exception of basal sediments in the point bar which a depth of ~ 70 cm. show poor sorting, probably reflecting mixing of sedi- The grain-size characteristics and selected aspects of ment in an energetic channel environment. It may be the geochemistry of sediment cores in the vicinity of a surmised that the abrupt change in sorting in this core mangrove tidal channel meander bend in Malaysia high- may pinpoint the time of the meander cut-off. Fleming light an abrupt change in channel hydrodynamic and (2017) identified a tendency for sorting to be homoge- sedimentation regimes following a meander cut-off. The neous in sediment composed of a similar range of size, straight, nearly 1 km-long cut-off channel (Fig. 1) points whereas poor sorting implies sediment mixing with dif- out to a rather abrupt hydrological change, rather than ferent sizes of sediment. being the product of the slow progressive rapproche- The abrupt variation in grain size and sedimentation ment of two opposite channel segments that eventually rate in the meander cut-off is also consistent with an in- coalesced to form a cut-off. A scrutiny of available aerial crease in OC, thus, further reflecting the consequences photographs and Google Earth images shows that the of this event on local sedimentation. The grain size in- Kerteh cut-off has been extant over at least the last 13 fluence on channel meander evolution is embedded not years. The cut-off may well have occurred during the big only in these changes in OC but also in heavy metals. flood event that hit Terengganu nearly a century ago Both particle size distribution and OC concentrations from 21 to 29 December 1926, accompanied by excep- changed significantly at ~ 70 cm, pointing to a change in tional rainfall (1944 mm compared to the annual mean the depositional environment. Meander cut-offs in tidal of ~ 2800 mm), and resulting in severe environmental flats have been shown to be depocenters of organic mat- damage and the destruction of thousand hectares of for- ter following the instauration of quiescent hydrodynamic ests (Chan 2012; Williamson 2016). conditions, as flow becomes diverted through the cut- Whatever the true age of the cut-off, the change en- off, and the abandoned meander belt may also become gendered by this event is recorded in the grain-size and progressively covered by vegetation (Anthony et al. TOC trends. Sediment particle characteristics have been 1996). This situation also concerns the Kerteh meander shown to provide information on the relationship be- belt which appears to be progressively invaded by man- tween hydrodynamics and transport or deposition (e.g., groves (Fig. 1). This increase in OC has also been shown Droppo et al. 2015). The dominantly fine-grained nature to go with a decrease in grain size (Sutherland 1999). of the sediments found in KR1 and KR2, composed es- Generally, OC binds more easily with fine sediment, sentially of silts and, to a lesser degree, clays (Figs. 2, 3), clay, or silt. reflects both the high degree of weathering of tropical Previous studies have shown that the distribution of soils and the effect of mangrove swamps on dampening heavy metals in sediment is closely related to organic water flows and favoring the accumulation of mud. Not- matter and mostly depends on the type of sediment withstanding, differences in grain size have been found (Karbassi et al. 2005; Abdul Razak et al. 2018). The between the two cores in the Kerteh swamp. Core KR2 metal concentration in the point bar is almost uniform shows, in particular, a very sharp transition from coarse, from the bottom to the top of the core, whereas the con- through fine to medium sand, at a depth of about 80 cm, centration in the cut-off shows a clear transition phase to relatively homogeneous silt and clay in the rest of the at a depth of 70 cm, thus, further providing evidence for upper part of the core. We attribute this sharp change to a morpho-sedimentary change in the Kerteh channel as- the transition from an active channel meander to me- sociated with meander cut-off. The abandonment of the ander cut-off. Meander cut-offs in tidal environments meander created conditions for the active accumulation have been shown to exhibit sharp upward changes in fa- of elements and OC in the cut-off, compared to the cies following abandonment (e.g., Anthony et al. 1996). point bar. Louma (1990) has shown this relationship be- The coarse basal deposits correspond to sand deposited tween the concentration of elements and sedimentation in an active channel bed. Once the cut-off occurred, the processes. The variability in metal concentration has also diversion of flows through the straight new channel was been shown to depend on the size and texture of sedi- accompanied by a shift to fine-grained deposits associ- ments (Jicknells and Kump 1984; Ramos et al. 1994), ated with settling in a low-energy, abandoned channel with larger mean grain size being associated with a lar- environment. The grain size in KR1 is relatively more ger variation of metal concentration at the same area homogeneous, though the upper part of the core (above (Morse et al. 1993). 70 cm) shows a clearly overall fining trend. This charac- In order to further clarify the relationships in the grain teristic may reflect a shift from relatively more coarser- size and geochemistry of these deposits, we used a cor- grained sedimentation at the base to more continuously relation coefficient matrix for each core (KR1: Table 2; finer-grained point-bar sedimentation. The differences KR2: Table 3). The correlation of metals with mean Shaari et al. Progress in Earth and Planetary Science (2020) 7:46 Page 8 of 11

Table 2 Correlation coefficient between MGS, TOC, and Table 4 Varifactors of varimax rotated loading results selected metals in core KR1 Variable KR1 KR2 MGS TOC Fe Na Mg Mn Ba Sr VF1 VF2 Communality VF1 VF2 Communality MGS 1 Depth −0.07 −0.82 0.68 −0.91 0.23 0.88 TOC 0.71 1 MGS 0.18 0.75 0.60 0.83 −0.48 0.92 Fe 0.60 0.71 1 TOC 0.14 0.90 0.84 0.90 −0.32 0.92 Na 0.30 0.30 0.59 1 Fe 0.54 0.71 0.81 0.90 −0.38 0.96 H Mg 0.46 0.50 0.76 0.58 1 Na 0.58 0.34 0.50 0.80 −0.49 0.88 L Mn 0.18 0.20 0.48 0.41 0.73 1 Mg 0.89 0.37 0.93 0.69 −0.69 0.97 Ba 0.27 0.36 0.53 0.40 0.78 0.56 1 Mn 0.79 0.09 0.64 0.65 −0.70 0.93 Sr 0.24 0.15 0.42 0.41 0.74 0.56 0.59 1 Ba 0.75 0.30 0.66 0.25 −0.93 0.94 H highest correlation, L lowest correlation Sr 0.86 −0.05 0.75 0.36 −0.89 0.93 grain size and TOC in cut-off was relatively stronger % Variance 38.3 32.8 54.7 38.3 compared to the point bar. The highest correlation value of the latter is 0.76 (Fe-Mg), followed by Fe-Cu (r = mean grain size (0.75) and total organic content (0.90). 0.75), Cu-Sr (r = 0.74), Mg-Mn (r = 0.73), mean Meanwhile, for KR2, varifactor VF1 explained 54.7% of sediment-TOC (r = 0.71), and TOC-Fe (r = 0.71). The the total variance with strong loadings on MGS (0.83), highest correlation value of the cut-off is r = 0.98 (Mg- total organic carbon (0.90), and metal, namely, Fe (0.90) Mn), whereas the lowest correlations occur between Ba- and Na (0.80), whereas varifactor VF2 accounted for Fe ( = 0.59) and Ba-TOC (r = 0.53), although both are 38.3% with negative loadings exhibited by the concentra- still considered as strongly correlated to each other. The tions of Ba and Sr. The PCA shows that only Fe and Na strong positive correlation between all parameters in the are influenced by changes in MGS and organic content. cut-off suggests that grain size is an important criterion In this study, communalities of the variance explained a for the attachment of metals and TOC under the condi- higher value for KR2 (0.88-0.97) than KR1 (0.50-0.93), tions of reduced flow energy that ensued following me- revealing that the extracted factor fitted well with factor ander abandonment. Nguyen et al. (2005) have shown solution. that highly correlated metals exhibited a similar behavior It is noteworthy that a distribution pattern of metal in the study area. Generally, the contents decreased as content versus sample depth was clearly observed in grain size increased. KR2 (Fig. 5b) compared to KR1 (Fig. 5a). Two clusters Principal components analysis (PCA) highlighted two were formed in KR2 in which metal accumulation in the significant compounds (eigenvalue > 1) that explained upper depths had a higher concentration. The relation- 71.2% (KR1) and 93.0% (KR2) of the total variance of the ship between metal (Fe, Na) and MGS or organic con- datasets (Table 4). For KR1, varifactor VF1 explained tent was much stronger for KR2 than KR1. This was 28.3% of the total variance in the datasets with positive highlighted by a straight line with a similar direction on loadings on metal content such as Mg (0.89), Mn (0.79), the biplot graph (not shown). We deduce from this the Ba (0.75), and Sr (0.86). Varifactor VF2 accounted for relationship that the accumulation of finer-grained sedi- 32.8% of the total variance with positive loadings on ments following meander cut-off is favorable to binding between metals and organic matter. The long-term (multi-decadal) mobility of mangrove Table 3 Correlation coefficient between MGS, TOC, and tidal channels has been shown to depend essentially on selected metals in core KR2 sediment inputs into the system, which alters the mor- MGS TOC Fe Na Mg Mn Ba Sr phodynamics of meanders, thus eventually generating, MGS 1 over even longer timescales (secular to millennial) the TOC 0.89 1 gradual reworking of mangrove tidal flats (Anthony Fe 0.92 0.94 1 2004). Active meander belt reworking in mangrove Na 0.88 0.88 0.89 1 swamps occurs where high sediment loads in channels, notably bedload, induces instability in flow conditions. Mg 0.92 0.86 0.91 0.89 1 This has probably been the case in the Kerteh channel, Mn 0.89 0.83 0.89 0.84 0.98H 1 given the “abrupt” morphology of the meander cut-off L Ba 0.66 0.53 0.59 0.68 0.83 0.81 1 (Fig. 1). The foregoing study of sediments associated Sr 0.74 0.63 0.66 0.72 0.86 0.85 0.90 1 with the tidal channel in the Kerteh Sungai mangrove H highest correlation, L lowest correlation swamp has shown that channel dynamics can be Shaari et al. Progress in Earth and Planetary Science (2020) 7:46 Page 9 of 11

Fig. 5 Loading plot of selected metals concentrations vs depth on KR1 (a) and KR2 (b) Shaari et al. Progress in Earth and Planetary Science (2020) 7:46 Page 10 of 11

important in generating variability in the sedimentology Anthony E.J. contributed on conceptual ideas and proof outline. He verified and geochemistry of mangrove sediments. The reasons the data and wrote the important part of the discussion with input from all authors. for the formation of a meander cut-off in this swamp are The author(s) read and approved the final manuscript. not clear. However, since its formation, the Kerteh chan- nel and its meander cut-off have exhibited apparent sta- Funding bility at the timescale of available satellite images and This work was supported by the Research Acculturation Grant Scheme (RAGS) [Vot. 57097, 2013]. aerial photographs covering the study area (13 years).

This stability suggests that the swamp and its channel Availability of data and materials network may now be largely in equilibrium with flow The datasets used and/or analyzed during the current study are available conditions. from the corresponding author on reasonable request.

Competing interests 4 Conclusions The authors declare that they have no competing interests. This study has shown that geomorphic changes in man- Author details grove tidal channels can generate significant variations 1Faculty of Science and Marine Environment, Universiti Malaysia Terengganu, in sediment characteristics and geochemistry. The data 21030 Kuala Nerus, Terengganu, Malaysia. 2Institute of Oceanography and from two cores in contrasting local geomorphic situa- Environment, Universiti Malaysia Terengganu, 21030 Kuala Nerus, Terengganu, Malaysia. 3Institute of Earth Science, College of Ocean Science tions associated with channel meandering show clear and Resource, National Taiwan Ocean University, Keelung, Taiwan. 4Center of trends in metal concentrations, MGS and sorting, and Excellence for Ocean Engineering, College of Engineering, National Taiwan TOC. The most noteworthy aspect highlighted by the Ocean University, Keelung, Taiwan. 5Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia. 6Faculty of study is that abrupt changes in channel flow conditions Bioengineering and Technology, Universiti Malaysia Kelantan, 17600 Jeli, generated by a meander cut-off can induce marked Kelantan, Malaysia. 7Department of Fisheries, Faculty of Agriculture, changes in the sedimentology and geochemistry of sedi- Universitas Gadjah Mada, Jl. Flora Gd. A4, Bulaksumur, Yogyakarta 55281, Indonesia. 8UM 34 CEREGE, CNRS, IRD, INRA, Collège de France, Aix Marseille ments over time. Spatio-temporal changes in the dynam- University, Aix-en-Provence, France. 9USR LEEISA, CNRS, Cayenne, French ics of tidal channels in mangrove swamps are, thus, an Guiana, France. important mechanism in generating variability in the Received: 5 May 2020 Accepted: 18 August 2020 long-term sedimentology and geochemistry of mangrove sediments. References Abbreviations Abdul Razak NS, Shaari H, Husain ML, Minhat FI, Mohd Azmi MF (2018) Vertical MGS: Mean grain size; ICP-MS: Inductively coupled plasma mass profile of heavy metals in mangrove and lagoon sediment cores of Sungai spectrometer; NBS: National Bureau of Standard; USDA: United States Kilim, Langkawi, Malaysia. Pollut Res 37(1):41–50 Department of Agriculture Allen JRL (2000) Morphodynamics of Holocene salt marshes: a review sketch from the Atlantic and Southern North Sea coasts of Europe. 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